A conventional wire-dot printing head is illustrated in FIG. 1 that shows a general arrangement of a prior art printing mechanism. Therein, designated at 101 is a wire-dot printing head having a printing wire (not shown), 102 is a carriage for supporting the wire-dot printing head 101, 103 and 104 are guide shafts for supporting the carriage 102 for movement in directions A and 105 is a platen for feeding printing paper. The carriage 102 moves in the directions A upon reception of signals from a spacing motor (not shown), thereby moving the wire-dot printing head 101 in the directions of the width of the printing paper, while the platen 105 rotates upon reception of signals from a line feed motor (not shown), thereby feeding the printing paper in a direction crossing the width direction. A printing operation can be carried out by the printing dot wire striking onto the printing paper at a position to be printed via, for example, an ink ribbon while the wire-dot printing head 101 is moved in the width direction of the printing paper at a predetermined speed. When the wire-dot printing head 101 has completed one line of printing after reaching an end position of the printing paper in the width direction, the platen 105 is rotated to feed the printing paper in the longitudinal direction thereof for a length sufficient for a new line of printing while the wire-dot printing head 101 returns to an original position, so that the printing wire starts to strike onto the area of the printing paper for the next line of pointing.
In a printer having a printing wire for striking onto the printing paper to effect the printing operation in such a manner, the force to be applied to the printing paper affects the printing quality and varies according to an interval (head gap g) between the wire-dot printing head 101 and the printing paper. There is provided in the prior art printer a manual lever (not shown) so that the head gap g can be varied depending on the thickness of the printing paper. Frequently there occurs an erroneous operation of the manual lever by an operator which causes an inferior printing operation. This results in a loss of printing paper and/or downtime of the printer.
There has been proposed an apparatus for automatically adjusting the head gap g without the need for manual adjustment. FIG. 2 shows a general arrangement of a printer having such automatic adjusting mechanism, FIG. 3 is an enlarged side elevational view of a main portion of FIG. 2, and FIG. 4 is a view illustrating a primary portion of the arrangement FIG. 3. In these figures, the elements that are the same as those of FIG. 1 are denoted as the same reference numerals.
In FIG. 2, the guide shaft 103 is attached to side frames 106, 107 via eccentric bushes 108, 109 which are rotatably supported by the side frames. As a means for rotating the eccentric bushes 108, 109, there is a mechanism as shown in FIGS. 2 and 4 in which the rotary drive force of a pulse motor 110 is transmitted to a gear 112 via a gear 111 provided on a rotary shaft of the pulse motor 110, to thereby rotate the eccentric bush 108 together with the gear 112. On the other hand, the carriage 102 has a holding portion 102a having a U-shape in cross section as shown in FIG. 3 and is movable in the directions of an arrow B to vary the head gap g. During an actual adjustment operation, the eccentric bushes 108, 109 are rotated until the wire-dot printing head 101 is brought into contact with the platen 105 to allow the pulse motor 110 to be in a step out state, and then the pulse motor 110 is rotated oppositely for a given pulse number corresponding to a desired head gap g.
However, inasmuch as the wire-dot printing head 101 is brought into contact with the platen 105 every time the head gap g is adjusted, there occurs the problem that adjustment of the head gap g takes substantial time. Further, the printing operation is slowed since the adjustment of the head gap g is carried out every time the printing paper is changed or is repeated every time the printing paper is changed in case a signal paper is inserted into the head gap g.
The head gap g is normally adjusted at one end portion of the platen 105 in view of the restricted adjustment time. Hence, no attention was paid to the possibilities of a variation of the head gap g caused by an error of the diameter of the platen 105, a deflection of the guide shaft 103 or the platen 105 or an eccentric phase difference of the eccentric bushes 108, 109. There are generated other problems in that the head gap g cannot be correctly set if a step out position of the pulse motor 110 is varied due to a variation of the pressing force against the platen 105 caused by the step out torque of the pulse motor 110, a variation of the load torque of other ports of the transmission mechanism, a deformation of the platen 105 that is made of an elastic material, or flexibility of the supporting shaft of the platen 105, etc., at the time when the head gap g is adjusted on the basis of the step out position of the pulse motor.
Furthermore, it is impossible to set the head gap g in accordance with a partial variation of the thickness of the printing medium relative to a printing medium having a different thickness such as an envelope, a bank book or a printing medium having perforations.
Accordingly, it is an object of the present invention to provide a wire-dot impact printer capable of solving the problems of the prior art printer, and capable of carrying out an appropriate adjustment of the head gap to effect a printing operation with high quality.